Adsorption, nanoporous materials structure

Gas, or vapor molecules, after the degasitication process, can go through the pore structure of crystalline and ordered nanoporous materials through a series of channels and/or cavities. Each layer of these channels and cavities is separated by a dense, gas-impermeable division, and within this adsorption space the molecules are subjected to force fields. The interaction with this adsorption field within the adsorption space is the base for the use of these materials in adsorption processes. Sorption operations used for separation processes imply molecular transfer from a gas or a liquid to the adsorbent pore network [2],... 

P.I. Ravikovitch and A.V. Neimark, Relations between Structural Parameters and Adsorption Characterization of Templated Nanoporous Materials with Cubic Symmetry. Langmuir, 2000, 16, 2419-2423. 

Nanoporous materials with channels and cavities of molecular dimensions have a number of potential applications, for example, in catalysis, ion-exchange, gas adsorption, etc. Of the amine-templated transition-metal phosphates, the zinc phosphates have the richest structural chemistry and show a variety of materials with different structural dimensionality. Jensen and coworkers [222] prepared amine-templated zinc phosphates that are an interesting example of the correlation between the temperature for the synthesis and die density/structural dimensionality of the synthetic products that may be obtained. 

Compared with the traditional adsorbent such as activated carbon, zeolites, and silica gel, electrospun nanoflbers are good candidates for heavy metal ion adsorption due to its large surface area, tailored pore structure, good interconnectivity of pores, and potential to incorporate active chemistry or functionality on nanoscale [62,63]. Moreover, recycle is of great importance in the field of water treatment taking this aspect into consideration, the nanofiber-based adsorbents are more suitable compared with powdered nanoporous materials. 

Pores are classified by the International Union of Pure and Applied Chemistry (lUPAC) by pore size as micropores (<2 nm), mesopores (2-50 nm) and macropores (>50 nm). Micropores are sometimes divided into ultramicropores (<0.7 nm) and supermicropores (1.4—2.0 nm). The terms nanopore and nanoporosity are not defined precisely but refer to nanometre-sized pores. Characterisation of the porous structures of materials is difficult because some MOF materials are flexible. A variety of isotherm equations and adsorptives have been used to characterise porous structures using gas adsorption techniques. Porous structures are characterised by surface areas [determined using Langmuir, Bmnauer-Emmett-Teller (BET), Dubinin-Radushkevich (DR), etc., equations], pore volumes [total, micropore (DR), etc.] and pore size distributions. 

Solvation behavior can be effectively predicted using electronic structure methods coupled with solvation methods, for example, the combination of continuum solvation methods such as COSMO with DFT as implemented in DMoF of Accelrys Materials Studio. An attractive alternative is statistical-mechanical 3D-RISM-KH molecular theory of solvation that predicts, from the first principles, the solvation structure and thermodynamics of solvated macromolecules with full molecular detail at the level of molecular simulation. In particular, this is illustrated here on the adsorption of bitumen fragments on zeolite nanoparticles. Furthermore, we have shown that the self-consistent field combinations of the KS-DFT and the OFE method with 3D-RISM-KH can predict electronic and solvation structure, and properties of various macromolecules in solution in a wide range of solvent composition and thermodynamic conditions. This includes the electronic structure, geometry optimization, reaction modeling with transition states, spectroscopic properties, adsorption strength and arrangement, supramolecular self-assembly,"and other effects for macromolecular systems in pure solvents, solvent mixtures, electrolyte solutions, " ionic liquids, and simple and complex solvents confined in nanoporous materials. Currently, the self-consistent field KS-DFT/3D-RISM-KH multiscale method is available only in the ADF software. 

A question of practical interest is the amount of electrolyte adsorbed into nanostructures and how this depends on various surface and solution parameters. The equilibrium concentration of ions inside porous structures will affect the applications, such as ion exchange resins and membranes, containment of nuclear wastes [67], and battery materials [68]. Experimental studies of electrosorption studies on a single planar electrode were reported [69]. Studies on porous structures are difficult, since most structures are ill defined with a wide distribution of pore sizes and surface charges. Only rough estimates of the average number of fixed charges and pore sizes were reported [70-73]. Molecular simulations of nonelectrolyte adsorption into nanopores were widely reported [58]. The confinement effect can lead to abnormalities of lowered critical points and compressed two-phase envelope [74]. 

In Chapter 2, the structure of these materials and, in Chapter 3, the syntheses methods were described. In Figure 6.14, the adsorption isotherm of N2 at 77 K on the mesoporous molecular sieve MCM-41 is shown [67], The existence of capillary condensation is obvious from the isotherm. This fact implies the existence of pores in the mesopore range, that is, between 2 and 50 nm, which, in modern terms, is the nanoporous region [2], Capillary condensation in mesopores is generally associated with a shift in the vapor-liquid coexistence in pores in comparison with the bulk fluid. That is, a fluid confined in a pore condenses at a pressure lower than the saturation pressure at a given temperature, given that the condensation pressure depends on the pore size and shape, and also on the strength of the interaction between the fluid and pore walls [2,4,5,41],... 

However, a better structure designing requests a finer characterization of nanopores. We need to know structural features of nanopores as accurate as possible in order to develop the best nanostructured materials for the specific function. Nevertheless, nanopores are hidden in the bulk of solids. Consequently, established surface science tools cannot be directly applied to the nanopore characterization, leading to necessity of an inherent characterization method for nanopores on the basis of gas adsorption. This paper summarizes main characterization methods, which can be applied to nanopore systems, and essential roles of gas adsorption will be described. 

Water adsorption in carbon-slit nanopores has been studied in detail by Striolo et al.4S4 using GCMC calculations. This is one of the few studies that has considered water in atomically structured pores. The adsorption isotherms are calculated at various pore widths, and hysteresis is observed in adsorption/ desorption. Using their results they propose that for fluid separation or gas storage, narrow pores in materials with uniform pore distribution size should be designed. 

Surface and structural properties of nanoporous solids can be studied directly by employing modem techniques such as atomic force microscopy, electron microscopy, X-ray analysis and various spectroscopic methods suitable for materials characterization and surface imping [4]. In addition, these properties can be investigated by indirect methods such as adsorption [1, 11-13], chromatography [14, 15] and thermal analysis [16]. The quantities evaluated from adsorption, chromatographic and thermodesorption data provide information about the whole adsorbent-adsorbate system. These data can by used mainly to extract... 

Gas adsorption is an important method for characterization of nanoporous carbons because it allows for evaluation of the specific surface area, pore volume, pore size, pore size distribution and surface properties of these materials [1, 10-12]. Although various techniques for measurement of gas adsorption data and methods of their analysis pear to be well established, an accurate and reliable evaluation of adsorption properties is still a difficult task. This can be attributed to the inherent features of many porous carbonaceous materials, namely, to their strong surface and structural heterogeneity. The effects of structural and surface heterogeneity in adsorption on nanoporous carbons are often difficult to separate. 

Industrial applications of nanoporous carbons are based on both their porosity and surface properties, and consequently, their characterization is of great importance. The results presented here demonsfrate a great usefulness of gas adsorption measurements for the characterization of nanoporous carbons. Low-pressure measurements provide an opportunity to study the microporous structure and surface proptaties of these materials and to monitor changes in these properties that result fiom structure and surface modification. High-pressure adsorption data allow for a detailed characterization of mesoporous structures of carbonaceous porous materials, providing their surface areas and pore size distributions. 

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